A method for reducing a penetration load to be received when striking a staple is proposed (see, e.g., JP 2004-276312 A). According to this method, when stapling a plurality of sheets of paper, a penetration resistance at the time of striking the staple is reduced. Specifically, when stapling the sheets of paper duplicated or printed by an image forming apparatus, such as a copy machine, a printer, etc., a penetration resistance reducing agent is attached on a portion of the sheets of paper to be stapled, and then, the staple is struck into the portion of the sheets of paper to be stapled.
However, this technique requires an apparatus for attaching the penetration resistance reducing agent to the portion of the sheets of paper to be stapled, resulting in a cost increase. Thus, when binding sheets of paper having a high penetration resistance with a general stapler, a staple having a large wire diameter is used in view of reducing cost. In this case, to improve stiffness of the stapler, the entire size of the stapler is required to be increased. When using a staple having a large wire diameter in an electric stapler, power consumption increases.